Method and apparatus to control the density of products produced from an extrusion process

Abstract

Density of products produced from a foam extrusion process is controlled by measuring the thickness and the basis weight. The density is calculated and is compared to a target, or desired value, to produce an error signal. The error signal is used to control the density.

What is claimed is: 
     
       1. A method for continuously and automatically controlling the product from a foam extrusion process to maintain the density of said products at a pre-determined value, said process has an extruder wherein resin and additives are introduced and are heated, a blowing agent is then introduced, and is mixed with the resin and additives to form a mixture, said mixture is cooled and is extruded to produce said product; said method comprising: continuously measuring the weight per unit area of said product by a basis weight sensor and generating an electrical signal related thereto;   continuously detecting the thickness of said product by a thickness sensor and generating an electrical signal related thereto;   generating an electrical signal related to the calculated value of density of said product based upon the electrical signal related to said basis weight measured and the electrical signal related to said thickness detected;   comparing the electrical signal related to said calculated density value to an electrical signal related to said predetermined density value to produce an error signal; and   controlling the density of the product produced by said process based upon the error signal.   
     
     
       2. The method of claim 1 wherein said controlling step comprises: adjusting the amount of blowing agent introduced into the extruder.   
     
     
       3. The method of claim 2 wherein the electrical signal related to the calculated value of density is based upon an operation of: dividing the weight per unit area that is measured by the thickness that is detected.   
     
     
       4. An apparatus for continuously and automatically controlling products produced from a foam extrusion process to maintain the density and thickness of each of said products at a predetermined value, said process has an extruder wherein resin and additives are introduced and are heated, a blowing agent is then introduced into the extruder and is mixed with the resin and additives to form a mixture, said mixture is cooled and is extruded to produce said product; said apparatus comprising: basis weight sensor capable of measuring the weight per unit area of said product and generating an electrical signal related thereto;   thickness sensor capable of detecting the thickness of said product and generating an electrical signal related thereto;   a density calculator capable of calculating the value of density of said product from said above electrical signal and generating a signal corresponding to the calculated density value;   means for comparing said calculated density value to an electrical signal related to said predetermined density value to produce an error signal; and   means for controlling the density of the product produced by said process based upon said error signal.   
     
     
       5. The apparatus of claim 1 wherein said controlling means comprises means for adjusting the amount of resin and additives introduced into said extruder. 
     
     
       6. The apparatus of claim 1 wherein said controlling means comprises means for adjusting the temperature of said mixture. 
     
     
       7. The apparatus of claim 1 wherein said controlling means comprises means for adjusting the amount of blowing agent introduced into said extruder. 
     
     
       8. The apparatus of claim 7 wherein said basis weight sensor comprises a radiation source to one side of said product, capable of emitting a beam of radiation through said product; and a radiation detector to the other side of said product capable of detecting the amount of radiation transmitted through said product. 
     
     
       9. The apparatus of claim 8 wherein said thickmess sensor comprises a non-contacting caliper gauge. 
     
     
       10. The apparatus of claim 8 wherein said thickness sensor comprises a contacting caliper gauge.